CVE-2026-27902: Svelte XSS Vulnerability via HTML Comment Injection in SSR Error Boundary Hydration Markers
Summary
CVE-2026-27902: Svelte: XSS via HTML Comment Injection in SSR Error Boundary Hydration Markers. Affects svelte versions >= 5.53.0, < 5.53.5. Fixed in 5.53.5. How to fix ↓
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Background
Svelte is a modern JavaScript framework for building user interfaces. It compiles components into highly optimized JavaScript at build time, allowing for fast and efficient web applications. Svelte's server-side rendering (SSR) capabilities enhance performance and SEO. However, a recent vulnerability has been identified that affects how errors are handled in SSR, potentially exposing applications to cross-site scripting (XSS) attacks.
Vulnerability Detail
The vulnerability arises from the improper escaping of error messages generated by the 'transformError' function. When these messages are included in the HTML output without proper sanitization, it allows an attacker to inject malicious HTML or JavaScript code. If an attacker can control the content returned by 'transformError', they can exploit this flaw to execute arbitrary scripts in the context of the user's browser, leading to potential data theft or session hijacking.
How To Fix
Latest published version: 5.57.2 as of 23 Oct 2026
npm install svelte@5.57.2Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
5.53.5
This is the first release that contains the fix.
npm install svelte@5.53.5Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, ensure that any error messages returned by 'transformError' are properly sanitized before being rendered in the HTML. You can implement a custom error handling function that escapes potentially dangerous characters. For example:
function safeTransformError(error) {
return String(error).replace(/</g, '<').replace(/>/g, '>');
}
Conclusion
The identified vulnerability in Svelte (CVE-2026-27902) poses a risk of XSS attacks due to improper error message handling in SSR. It is crucial for developers using Svelte to update to the patched version 5.53.5 to secure their applications. For those unable to update immediately, implementing a custom error handling function can serve as a temporary measure.
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FAQ
The CVSS score of this vulnerability is 5.3, indicating a moderate severity level.
You can use Vulert's playground at Vulert Scanner to check if your application is affected by this vulnerability without needing to sign up.
If exploited, this vulnerability can lead to XSS attacks, allowing attackers to execute malicious scripts in users' browsers, potentially leading to data theft or session hijacking.
Yes, a patch is available in Svelte version 5.53.5. It is recommended to update to this version to mitigate the risk.
If immediate updating is not possible, implement a custom error handling function to sanitize error messages as a temporary workaround.
References
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